Movement in dynamic systems typically results in unwanted vibrations that are both mechanical and acoustic in nature. These vibrations can have a detrimental affect on the operation of such systems. One dynamic system that is particularly sensitive to unwanted vibrations is a computer disc drive.
In a computer disc drive, data is stored on discs in concentric tracks. In disc drives with relatively high track densities, a servo feedback loop is used to maintain a head over the desired track during read or write operations. This is typically accomplished utilizing prerecorded servo information on sectors that are interspersed along a disc. During track seeking operations, position information is sampled from the sectors as the head moves from a departure track to a destination track. During track following operations, successive servo samples are taken from the servo sectors of a single data track being followed by the head under servo loop control. The sampled positional information is then compared by the digital servo control system with predicted positional location, and any deviation or error results in a correction signal applied to an actuator and resultant corrective movement of the head-carrying actuator arm.
Historically, only one actuator, typically a voice coil motor (VCM), was used to position the head. Recently, micro-actuators have been proposed that would be used in combination with the VCM to position the head. Because they are small, such micro-actuators generally have a better frequency response than the VCM. As such, they are better able to follow high frequency control signals.
The mechanical structure of the disc drive comprises various resonant modes that cause actuator oscillation during track seeking/settling operations and even during track following operations. Currently, the most widely used technique to handle such unwanted vibrations is to employ notch filters at the mechanical resonance frequencies. To implement a notch filter, the position measurement sampling frequency has to be twice as high as the resonance frequency to be notched.
In disc drives with micro-actuators included on the actuator arm, the mechanical resonance frequency may be very high (44 Kilohertz (kHz)–51 kHz, for example) and therefore the position measurement sampling frequency to implement the required notch filter will have to be extremely high (about 100 kHz). However, the sampling frequency is limited by disc spin rate, the number of servo sectors and processor speed. Therefore, to implement such notch filters, significant processing speed and processing capability is usually required. Unfortunately, higher speed sampling devices needed to implement such notch filters usually come at a high premium in cost and perhaps are not even available at the sampling rates needed.
Embodiments of the present invention provide solutions to these and other problems, and/or offer other advantages over the prior art.